Numerous oscillator circuits have been designed which fundamentally involve an amplifier and a tuned circuit, and more particularly crystals, where the crystal, in essence, sets the operating frequency of the circuit. Many of these circuits now employ gate invertors, operational amplifiers or bipolar transistor amplifiers. These circuits require passive components, in addition to the crystal, in the feedback loop, to provide sufficient phase-shift in the feedback loop to create the proper condition for oscillation. Most of the prior art circuits have the amplifier operate in the flat portion of their gain versus frequency curve. In these circuits, therefore, a relatively high frequency amplifier, which is relatively more expensive than its low frequency counterparts must be employed. Moreover, in some instances, the designer does not have the option of selecting higher frequency amplifiers because the amplifier is included in a dedicated microchip, such as in a computer, for the purpose of developing a clock, and the designer only provides an external crystal to work in conjunction with the amplifier. In such an event, with a conventional oscillator design, the amplifier may limit the clock to a lower frequency of operation than desired.
One example of such an instance are CMOS microprocessor chips which include a CMOS inverter amplifier where an external crystal is to be added to provide a clock. Here the CMOS inverter design itself may limit the oscillator frequency below that desired when a conventional oscillator design is employed.
Furthermore, prior art crystal oscillators require a substantial number of extra components or circuitry to allow the use of crystals operating at their overtone or harmonic frequencies.